1. Field of the Invention
Embodiments disclosed herein generally relate to magnetic recording, and in particular, relates to a high-frequency oscillator for use in a recording method that magnetically records by simultaneously irradiating a high-frequency magnetic field in addition to a write magnetic field to induce magnetic resonance in a recording medium, specifically, to a microwave-assisted recording technology, a magnetic head installed with this element, and a magnetic recording device.
2. Description of the Related Art
Accompanying recent advances in the performance of computers and faster, higher capacity networks, the amount of information transmitted in the form of digital data has jumped dramatically. In order to efficiently receive, distribute, and extract these huge amounts of information, storage devices capable of high-speed input and output of huge amounts of information are needed. As high-density recording in magnetic disks has progressed, the gradual degradation of the recorded signals by thermal fluctuations has become a tangible problem. The cause is the reduction in the volumes of the crystal particles, as the magnetic recording medium is a collection of magnetocrystalline particles. To obtain satisfactory resistance to thermal fluctuations and stability, it is believed that the often used thermal fluctuation index Kβ (Kβ=KuV/kT; Ku: magnetic anisotropy, V: particle volume, T: absolute temperature, k: Boltzmann's constant) must be at least 70. If Ku and T (materials and environment) are constant, magnetic reversals caused by thermal fluctuations easily occur for particles having a smaller V. As high-density recording advances and the volume of the recording film occupied by one bit decreases, V must be reduced, and thermal fluctuations can no longer be ignored. If Ku is increased to suppress these thermal fluctuations, the magnetic field for reversing magnetization needed in magnetic recording exceeds the recording magnetic field that can be generated by the recording head, and recording becomes impossible.
One technique to avoid this problem is with Microwave-Assisted Magnetic Recording (MAMR). As shown by a MAMR head 100 in FIG. 1, by applying a high-frequency magnetic field generated by a spin torque oscillator (STO) 102 provided adjacent to an opposite magnetic pole 106 and a main magnetic pole 104, as well as a write magnetic field from the main magnetic pole of the vertical magnetic recording head to a magnetic recording medium 108 having high magnetic anisotropy, MAMR sets the area to be recorded in the magnetic resonance state and varies the magnetization in order to lower the magnetic field for magnetization reversal and record. Recording becomes possible in an area irradiated by microwaves in a magnetic recording medium suited for high-density recording exceeding 1 Tbit/in2, which is difficult to record when using a conventional magnetic head that has an inadequate recording magnetic field. MAMR obtains a large assist effect as the high-frequency magnetic field intensity becomes stronger, as it is a magnetic recording technique that induces magnetic resonance in the recording medium 108 and achieves magnetization reversal by using the write magnetic field and the high-frequency magnetic field, and is expected to be capable of recording high-Ku media suited for high recording densities.
When an STO 102 is used in an MAMR head 100, the high-frequency magnetic field is generated by alternating the input of magnetic charge on the surface of the magnetic field generating layer (FGL) 110 that reverses magnetization in the surface of the stacked layers due to spin torque actions. The method of antiferromagnetic (AF) mode oscillation, like the STO 102 of FIG. 1, holds the FGL 110 magnetization and the reference layer (RL) 112 magnetization in the anti-parallel state and rotates the magnetizations by the spin torque actions generated by electrical conduction in the high-frequency field generating layer 110 and reference layer 112 separated by non-magnetic spin conducting material 114, such as Cu, and enters the oscillation state, as shown in FIG. 2. The principle behind AF mode oscillation uses the fact that when the spin torque magnetic field Hstq-FGL applied to the magnetization mFGL of the FGL 110 and the spin torque magnetic field Hstq-RF applied to the magnetization mRL of the reference layer 112 are in the same direction, their magnitudes usually differ. Regarding FIG. 2, the relationship of Hstq-RF and Hstq-FGL is:
                              H                      stq            -            ref                          =                              (                                          m                FGL                            ×                              m                RL                                      )                    ⁢                      CJ                                          (                                                      B                    s                                    ⁢                  t                                )                            RL                                ⁢                      g            AF                                              Equation        ⁢                                  ⁢        1                                          H                      stq            -            FGL                          =                              (                                          m                FGL                            ×                              m                RL                                      )                    ⁢                      CJ                                          (                                                      B                    s                                    ⁢                  t                                )                            FGL                                ⁢                      g            AF                                              Equation        ⁢                                  ⁢        2            
where: mFGL and mRL represent the unit magnetization vectors of FGL 110 and RL 112, respectively, (Bst)FGL and (Bst)RL represent the products of the fitness thickness and the saturated magnetization of FGL 110 and RL 112, respectively, and J is the current density in the direction perpendicular to the stacked layer surface of the STO 102. The g-factor is a variable that depends on the polarizability P and the angle of magnetization, and is denoted by gAJ because in AF mode oscillation, FGL 110 and RL 112 are nearly anti-parallel. If (Bst)FGL>(Bst)RL is designed, then Hstq-FGL<Hstq-RF, and the movement of the magnetization of the tracking FGL 110 is delayed and cannot track the escaping RL 112 magnetization. However, if the spin torque magnetic field depends on the outer product of mFGL and mRL, the action disappears when the anti-parallel state is reached. Consequently, while the FGL 110 magnetization and the RL 112 magnetization are held in the near anti-parallel state, the periphery of the applied magnetic field continuously rotates. An advantage is that a large spin torque effect is extracted at a relatively small current because the g-factor becomes extremely large (gAF>>g) when the FGL 110 magnetization and the RL 112 magnetization enter the anti-parallel state.
In a prototype MAMR head with STO 300, FGL 310, RL 312, and non-magnetic spin conducting material 314, even for a thicker FGL film 310 with the objective of obtaining a stronger high-frequency magnetic field, the assist effect did not improve when some thickness was exceeded. As a result of detailed studies of the oscillation characteristics of the STO 300 by micro-magnetic simulation, the cause was determined to be the generation of a prominent domain having a magnetization component perpendicular to the film surface when the FGL 310 film thickened and some film thickness was exceeded. The magnetization emitted at the FGL-side 310 surface did not increase as the film thickness of the stacked layers increased, as shown by FIG. 3, and part of the FGL 310 turned into a dead layer 316. The reason is believed to be the exchange interaction in the film thickness direction, and easy magnetization perpendicular to the film surface.
Therefore, there is a need in the art for an MAMR magnetic recording head having a large assist effect by using an STO having a structure that obtains a strong high-frequency magnetic field, and a hard disk drive that uses this head.